1. Technical Field
This invention generally relates to computer system memory, and more specifically relates to ways of scrubbing computer system memory to correct errors.
2. Background Art
Since the dawn of the computer age, computer systems have evolved into extremely sophisticated devices that may be found in many different settings. Computer systems typically include a combination of hardware (e.g., semiconductors, circuit boards, etc.) and software (e.g., computer programs). One key component in any computer system is memory.
Modem computer systems typically include dynamic random-access memory (DRAM). DRAM is different than static RAM in that its contents must be continually refreshed to avoid losing data. A static RAM, in contrast, maintains its contents as long as power is present without the need to refresh the memory. This maintenance of memory in a static RAM comes at the expense of additional transistors for each memory cell that are not required in a DRAM cell. For this reason, DRAMs typically have densities significantly greater than static RAMs, thereby providing a much greater amount of memory at a lower cost than is possible using static RAM.
Modem DRAM memory controllers use a technique known as “scrubbing” to automatically detect and correct soft errors that may arise in memory that are typically caused by alpha particles. Sophisticated error correction circuitry has been developed that allow detecting any single-bit error, or any single-symbol error in a DRAM. A single-bit error occurs when one bit at a specified address changes state unexpectedly. A single-symbol error occurs when multiple bits within a packet (or symbol) at a specified address in a single DRAM chip change state unexpectedly. During a typical scrub cycle, a cache line is read, causing a corresponding read of the data from memory. The error correction circuitry detects if there are any single-bit or single-symbol errors, and if so, generates the correct data, which is then written to the cacheline, and in turn, back to the memory. In this manner, the scrub circuitry may successfully recover from any single-bit or single-symbol errors in memory. In many computer systems, the scrub circuitry tries to scrub all of system memory at least one each day. A brief description of memory scrubbing is found in IBM Enterprise X-Architecture Technology, First Edition 2002, pp. 19-21, which is incorporated herein by reference.
A scrub cycle typically occurs when there are no pending processor reads or writes to memory. Once the scrub cycle starts, the entire read-correct-write sequence is performed as an atomic operation to assure data integrity. If processor reads occur just after the read operation in the scrub cycle, the processor reads have to wait until the atomic read-correct-write scrub operation is performed. As a result, prior art memory controllers negatively affect system performance when performing scrub cycles. Without a way for performing scrub cycles in a way that does not make processor read cycles wait, the computer industry will continue to be plagued with decreased performance during scrub cycles.